Magnetic switches are used to make or break electrical connections using a local permanent and/or electromagnetic field. A “normally open” type of magnetic switch closes when brought into close proximity to a suitably oriented magnetic field, while a “normally closed” type opens when subjected to a magnetic field. Such switches may be used in a variety of industrial, medical, and security applications, and may be particularly advantageous in situations where opening or closing of a circuit may be accomplished without physical contact with the switch. For example, in-vivo medical devices may be sealed to provide biocompatibility and to protect the device. Such devices may not have an external “on-off” switch to activate the device. A magnetic switch sealed within the device and controlled by an external magnet can provide a switch to activate the device.
Many commercially available magnetic switches are based on “reed switches” constructed of thin elastic reeds made of a ferromagnetic material. These reeds may be tipped with noble metal films to provide low contact resistance and sealed into a glass and/or other tube. When a permanent magnet or electromagnet is brought into close proximity with the tube, the reeds either move toward or away from one another, making or breaking the contact. When the magnet is removed, the reeds return elastically to their original position, resetting the switch. One potential disadvantage of conventional reed-based magnetic switches is that they may be relatively large, for example about one inch in length and about ⅛″ to ¼″ in diameter. For applications where small size is desired, such as in-vivo medical devices, conventional reed magnetic switches may be too large. Moreover, reed switches may be undesirably fragile.
MEMS devices have been recently developed as alternatives for conventional electromechanical devices, in-part because MEMS devices are potentially low cost, due to the use of simplified microelectronic fabrication techniques. New functionality may also be provided because MEMS devices can be much smaller than conventional electromechanical systems and devices. MEMS devices are described, for example, in U.S. patent application Publication No. 2002/0171909 A1 to Wood et al., entitled MEMS Reflectors Having Tail Portions That Extend Inside a Recess and Head Portions That Extend Outside the Recess and Methods of Forming Same, and U.S. Pat. No. 6,396,975 to Wood et al., entitled MEMS Optical Cross-Connect Switch.
MEMS devices and manufacturing methods have been used to provide magnetic switches. For example, Integrated Micromachines Inc. (IMMI) developed a reed-like magnetic switch using MEMS technology. See FIG. 1. It is a normally open switch with approximate dimensions 2.5×2×1 mm and contact resistance in closed state of about 50 Ω. Unfortunately, the reed configuration may inherently lead to poor shock/vibration resistance and/or high contact resistance. It also may be difficult to build a normally closed switch based on this technology. The switch also may only be configured as Single Pole Single Throw (SPST), but it may be difficult to provide Double Pole Single Throw (DPST) or Single Pole Double Throw (SPDT) versions. Reed switches also generally do not have a wiping action, i.e., they generally are not self-cleaning and contact resistance may go up with time.
Published U.S. patent application Publication No. 2002/0140533 A1 to Miyazaki et al., entitled Method of Producing An Integrated Type Microswitch, also describes a MEMS-based microswitch. As described in the Abstract of this patent application publication, an integrated type microswitch with high durability is provided. The integrated type microswitch is of the construction through micro-machining process in which a movable plate is provided above a fulcrum means movable in seesaw movement by means of either electrostatic or magnetic force, so that either one of movable contacts mounted on opposite free ends thereof is on-off connected to fixed contact disposed in opposite relation due to seesaw movement of the movable plate. See the Abstract of this publication.
U.S. Pat. No. 6,320,145 to Tai et al., entitled Fabricating and Using a Micromachined Magnetostatic Relay or Switch, also describes a MEMS-based microswitch. As described in the Abstract of this patent, a micromachined magnetostatic relay or switch includes a springing beam on which a magnetic actuation plate is formed. The springing beam also includes an electrically conductive contact. In the presence of a magnetic field, the magnetic material causes the springing beam to bend, moving the electrically conductive contact either toward or away from another contact, and thus creating either an electrical short-circuit or an electrical open-circuit. The switch is fabricated from silicon substrates and is particularly useful in forming a MEMs commutation and control circuit for a miniaturized DC motor. See the Abstract of this patent. A similar configuration is described in a publication entitled Micromachined Magnetostatic Switches, to Tai et al., Jet Propulsion Laboratory, California Institute of Technology, October 1998, pp. i, 1-7, 1b-3b.
A MEMS micromagnetic actuator is also described in U.S. Pat. No. 5,629,918 to Ho et al., entitled Electromagnetically Actuated Micromachined Flap. As noted in the Abstract of this patent, a surface micromachined micromagnetic actuator is provided with a flap capable of achieving large deflections above 100 microns using magnetic force as the actuating force. The flap is coupled by one or more beams to a substrate and is cantilevered over the substrate. A Permalloy layer or a magnetic coil is disposed on the flap such that when the flap is placed in a magnetic field, it can be caused to selectively interact and rotate out of the plane of the magnetic actuator. The cantilevered flap is released from the underlying substrate by etching out an underlying sacrificial layer disposed between the flap and the substrate. The etched out and now cantilevered flap is magnetically actuated to maintain it out of contact with the substrate while the just etched device is dried in order to obtain high release yields. See the Abstract of this patent.
Finally, an implantable medical device that includes a MEMS magnetic switch is described in U.S. Pat. No. 6,580,947 to Thompson, entitled Magnetic Field Sensor for an Implantable Medical Device. As described in the Abstract of this patent, an implantable medical device (IMD) uses a solid-state sensor for detecting the application of an external magnetic field, the sensor comprises one or more magnetic field responsive microelectromechanical (MEM) switch fabricated in an IC coupled to a switch signal processing circuit of the IC that periodically determines the state of each MEM. The MEM switch comprises a moveable contact suspended over a fixed contact by a suspension member such that the MEM switch contacts are either normally open or normally closed. A ferromagnetic layer is formed on the suspension member, and the suspended contact is attracted or repelled toward or away from the fixed contact. The ferromagnetic layer, the characteristics of the suspension member, and the spacing of the switch contacts may be tailored to make the switch contacts close (or open) in response to a threshold magnetic field strength and/or polarity. A plurality of such magnetically actuated MEM switches are provided to cause the IMD to change operating mode or a parameter value and to enable or effect programming and uplink telemetry functions. See the Abstract of this patent.